1. Field of the Invention
The present invention relates to a method of and an apparatus for playing back a recordable optical disc, and more particularly to a method of and an apparatus for reproducing recorded information from a recordable optical disc while other information is being recorded thereon.
2. Description of the Prior Art
Heretofore, there are known compact discs (CD) having diameters of 12 cm and 8 cm. As shown in FIG. 5 of the accompanying drawings, a compact disc D.sub.0 has a lead-in area d.sub.1, a program area d.sub.2, and a lead-out area d.sub.3 that are arranged successively from an innermost circular boundary on the disc. The compact disc stores a digital signal in the form of an EFM (Eight to Fourteen Modulation) channel code. The recorded digital signal includes a main code representing main information such as music information and a subcode such as a time code (Q data) representing time information. The EFM signal contains a clock signal. The clock signal is compared with a reference synchronizing signal, and any error is fed back through an error amplifier and a spindle motor driver amplifier to a spindle motor for controlling the rotational speed of the spindle motor. Such a motor control process is referred to as a spindle-motor servo control process.
The lead-in area d.sub.1 stores index information called TOC (Table of Contents) information. The TOC information contains a subcode indicating the total number of music pieces recorded in the program area, the total playback time of recorded music pieces, and other information. As shown in FIG. 6 of the accompanying drawings, the program area contains a main code representative of music information, Q data of the subcode, track numbers (TNO) indicating the numbers of music pieces, relative playback times (P-TIME) from the start of the tracks, an absolute total playback time (A-TIME) measured from the track number 1, and other information. The lead-out area contains a lead-out code representing the lead-out area.
The CDs with such a recording format are a medium used for playback only. There has recently been developed a recordable compact disc (R-CD) D.sub.1 which has substantially the same format as the conventional CD D.sub.0 and which allows additional information to be recorded subsequently. The recording format of the R-CD D.sub.1 is referred to as "Orange Book". While the recording format of the R-CD D.sub.1 is similar to that of the ordinary CD D.sub.0, it additionally has a PCA d.sub.x (Power Calibration Area) and a PMA D.sub.y (Program Memory Area) that are positioned radially inwardly of the lead-in area d.sub.1, as shown in FIG. 7 of the accompanying drawings. The PCA d.sub.x and the PMA d.sub.y will be described in detail later on.
Tracks on the R-CD D.sub.1 are associated with guide grooves that are wobbled or made tortuous by the frequency of a carrier of 22.05 KHz which is frequency-modulated by data indicating absolute time information (ATIP: Absolute Time In Pregroove). A laser beam for recording desired information on the R-CD D.sub.1 is controlled to follow the guide grooves under tracking control, and the rotational speed of the spindle motor which rotates the R-CD D.sub.1 is controlled so that the wobbling guide groove pattern has a central frequency of 22.05 KHz. In this manner, tracking servo and spindle-motor servo processes can be effected on unrecorded R-CDs as well as recorded R-CDs.
The ATIP information will be described below. The ATIP information contains a plurality of frames each composed of 42 bits. The 42 bits of each frame include 4 bits indicative of a sync pattern (SYNC), 8 bits indicative of a minute (MIN), 8 bits indicative of a second (SEC), 8 bits indicative of a frame (FRAME), and 14 bits indicative of an error correcting code (ECC). The absolute time information represented by the ATIP information equals 1 second in 75 frames as is the case with the Q data of the subcode.
In addition to the ordinary absolution time information, the ATIP information includes special information encoded by the combination of the most significant bits (MSB) of the 8-bit data which indicate MIN, SEC, and FRAME, i.e., the combination of the 5th, 13th, and 21th bits as counted from the beginning of the frame. If the combination of the 5th, 13th, and 21th bits is "101", then the information represented by the 6th, 7th, and 8th bits indicates a recommended recording power, i.e., a recording laser beam intensity, for use with the disc. There are eight recommended recording powers indicated by eight combinations of the 6th, 7th, and 8th bits, ranging from "000" to "111". The recommended recording powers are determined under the conditions that the recording laser beam has a wavelength .lambda. of 780 nm and is generated at a temperature T of 25.degree. C. Since the wavelength .lambda. is temperature-dependent and different objective lenses have different aperture ratios NA, an optimum recording power may not necessarily be the same as any of the recommended recording powers.
The PCA d.sub.x will now be described below. The PCA d.sub.x is an area where test recording is carried out prior to an information recording process, so that the power of the recording laser beam will be optimized for recording desired information in the information recording process. The PCA d.sub.x is composed of a count area and a test area. The count area has a plurality of subareas corresponding respectively to a plurality of subareas of the test area, and each containing a suitable EFM signal recorded therein. Test recording for measuring an optimum recording laser beam power is carried out in the subareas of the test area. This is because the recommended recording powers indicated by the ATIP information may not necessarily be the same as an optimum recording power. One of the subareas of the test area is consumed by a single test recording cycle.
Any subarea of the test area which has once been used by test recording is not available for a next test recording cycle. It is therefore necessary to search for an unused subarea positioned radially inwardly of the used subarea for use in the next test recording cycle. The count area is provided to facilitate the searching process. More specifically, if suitable EFM signals have been recorded in certain subareas of the count area and no EFM signal has been recorded in a subarea next to those subareas, then unrecorded subarea indicates that the corresponding subarea of the test area is available for test recording. Consequently, that corresponding subarea of the test area may be searched for and test recording carried out therein for measuring an optimum recording laser beam power. After an optimum recording laser beam power has been measured in the subarea of the test area, the corresponding subarea of the count area is searched for, and suitable EFM signal is recorded in that subarea, indicating that test recording has already been conducted in the subarea of the count area.
The PMA d.sub.y is an area for storing a recording history of items of information successively recorded in the program area. Specifically, the start addresses and end addresses of track numbers recorded in the program area, for example, are recorded in the PMA d.sub.y in the same format as the TOC information in the lead-in area d.sub.1.
The recording history of successively recorded items of information is stored in the PMA D.sub.y for the following reasons: On a partially recorded disc, information may further be recorded in the remaining blank of the program area. Therefore, the information in the PMA (hereinafter referred to as "temporary TOC information") cannot be recorded in the lead-in area d.sub.1 until the completion of recording of all the desired information is finally indicated. Consequently, information about the tracks that have already been recorded is temporarily stored in the PMA d.sub.y. When the user or controller of the disc indicates that no more information will be recorded, the TOC information and a lead-out signal are recorded for the first time. At the time the recording is completed, the TOC information in the PMA is transferred to the lead-in area d.sub.1 a plurality of times for reliability. The finalized disc (R-CD), on which the recording of all the necessary information is completed, is now based on the CD format, and can be played back by an ordinary playback-only disc player which does not have an ATIP decoder for decoding the ATIP information. FIGS. 8(A), 8(B), and 8(C) show the manner in which information is recorded on an R-CD. Specifically, FIG. 8(A) show an unrecorded R-CD (virgin disc), FIG. 8(B) a partially recorded R-CD, and 8(C) a finalized R-CD.
As described above, no information is recorded in the lead-in area of a partially recorded R-CD. Since no EFM signal is obtained from lead-in area when the R-CD is played back by the ordinary playback-only disc player while the optical pickup of the disc player is positioned over the lead-in area, the spindle-motor servo control process cannot be locked over the lead-in area, and the spindle motor tends to rotate out of control, e.g., to rotate at unduly high speed or in the opposite direction. It is impossible to perform the spindle-motor servo control process using the wobbling frequency of the guide groove on the R-CD because the ordinary playback-only disc player has no means for detecting the wobbling frequency.